Field of the Invention
The invention concerns a method and device for driving a capacitive control element, especially a piezoelectrically operated fuel injection valve of an internal combustion engine.
Piezo control elements are made from a number of piezoceramic layers and form a "stack" that changes dimensions when a voltage is applied, especially the stack's length s by a deviation ds, or generates an electrical voltage during mechanical compression or tension.
The electrical properties of such a piezostack vary with temperature to which it is exposed. With increasing temperature, its capacitance is increased, but so is the deviation. At the temperatures to be considered for automotive applications of about -40.degree. C. to +140.degree. C., changes of up to a factor of 2 are observed,
In Published, Non-Prosecuted German Patent Application 196 44 521.3 A, corresponding to U.S. patent application Ser. No. 09/299,367, it was already proposed to drive the capacitive control element with constant energy because loading with constant energy produces a much more constant deviation over the required temperature range.
The deviation varies roughly linearly with an applied voltage at a specified control element capacitance or a specified temperature. If the temperature varies so does the deviation at equal voltage. On the other hand, the deviation varies in proportion to the square of the applied energy (ds-e.sup.2), but independently of temperature.
It is very complicated to supply a specified amount of energy to a control element. In the object of the German Patent Application 196 44 521.3 A, current and voltage must be measured, the product integrated from this and the charging process interrupted when the integral value reaches a stipulated value e=.intg.uidt. A simplification is obtained if the control element is charged with a constant current. Multiplication is then unnecessary.